1. Field of Invention
This invention relates to ultrasonic imaging and, more particularly, to a method and apparatus for ultrasonically imaging small cavities.
2. Related Art
Ultrasonic imaging devices are used to obtain a visual image of the inner walls and features of a blood vessel for diagnostic purposes. For example, ultrasonic imaging is used to determine the location of a stenotic lesion or stenosis. In addition, ultrasonic transducers are incorporated into interventional devices such as balloon dilation catheters for use in percutaneous transluminal coronary angioplasty (PTCA) to allow imaging and other procedures to be performed with a single instrument.
An ultrasonic image is obtained by inserting a catheter having an ultrasonic transducer at its tip into a blood vessel. Such a transducer typically has a number of piezoelectric elements or other acoustic elements arranged coaxially in a ring around a central guidewire lumen. A computer system individually controls the generation and reception of ultrasonic waves from each element through integrated microcircuits in the catheter tip. The ultrasonic waves reflect off the inner walls and features of the blood vessel, and the transducer elements receive the reflected waves and output an electrical signals in response. The computer system receives the electrical signals from each element, processes the signals, and assembles the processed signals into a digital image for output to a display.
In the displayed image, a visual artifact or blind spot occurs in regions near the elements, and this artifact is commonly referred to as a "ringdown" artifact. The ringdown artifact occurs because the same element both transmits and receives the ultrasonic waves. To generate an ultrasonic wave, an electrical pulse is applied to an element which causes that element to vibrate. After generating the desired ultrasonic wave, the element continues to vibrate or oscillate until the oscillations damp out. This damped oscillation causes the element to generate an electrical signal which is commonly referred to as a "ringdown" signal. The time required for the element to stop vibrating is called the ringdown time. Even during the ringdown time, the element is also being used to "listen" for or sense the echoes from the ultrasonic waves reflecting off tissues.
Initially, the ringdown signal generated by the element is generally a much stronger signal than the signal generated by an echo of the ultrasonic wave. In fact, the ringdown signal can be as much as 80 dB larger than the echo signal.
Because the amplitude of the ringdown signal is so large relative to the echo signal, the ringdown signal saturates the front-end amplifiers of the imaging device circuitry and thus create artifacts in the image. This saturation of the amplifiers effectively creates the blind spot which shows up as a corona in the generated image in an area immediately adjacent the surface of the transducer.
U.S. Pat. No. 5,183,048 to Eberle teaches a method of removing the ringdown signal and reducing artifacts in the displayed image by subtracting a reference waveform corresponding to the ringdown waveform from the imaging data from the elements. The reference waveform is generated or acquired prior to starting the imaging process. It may either be acquired outside the body by placing the catheter in water, or it may be acquired in vivo by placing the catheter in a large vessel to obtain an echo-free waveform.
During normal operation, the ringdown signal drifts in both phase and amplitude over time with respect to the reference waveform, and consequently the reference waveform may not properly compensate for ringdown drift. Many factors affect ringdown drift. One identified source of ringdown drift is temperature change. The temperature of the probe changes during normal operation as the electronics generate heat. Blood flow around the probe also affects the temperature because the blood can act as a coolant. If the blood flow decreases, less heat is removed from the probe and the temperature increases.
The ringdown drift degrades the quality of the image near the tip of the catheter because the reference ringdown waveform no longer reflects the most recent ringdown signal. To compensate for ringdown drift, a new reference ringdown waveform can be generated in a large vessel. However, this method is time consuming and requires repositioning of the catheter.
U.S. Pat. No. 5,601,082 to Barlow et al. teaches a method of removing ringdown drift in which a reference scan is generated and updated it on the basis of a long term running average, then subtracted from a current scan to remove the ringdown. However, that method has also been found to remove the desired tissue echoes or data from the image.